This invention relates to semiconductor devices, and more particularly to metal contacts and interconnections for semiconductor integrated circuits.
In the manufacture of VLSI devices, a thin metal coating such as aluminum is deposited and patterned to create contacts and interconnections. Problems occur in thinning of the metal at near-vertical sidewalls of holes in thick insulator layers where the metallization pattern makes contact to silicon regions or to other layers such as polysilicon. Thinner metal at these sidewalls or other steps results in higher resistance and a propensity for electro-migration failures. Heretofore, the steepness of the sidewalls has been minimized by a "reflow" process, but this necessitates undesirable high temperature operations and larger geometries.
In particular, the shrinking dimensions of VLSI devices such as the 1-Megabit dynamic RAM, requiring one micron sized geometries, creates fundamental problems with forming good metalization to silicon contacts. The proximity to unrelated structures reaches the point where reflowing the insulating multilevel oxide, or sloping the contact edges in any other fashion, is becoming unacceptable. However, sputtered aluminum is unacceptably thin on the edge of a vertical contact. Various methods have been tried to resolve this problem, including using lower resistance materials beneath the insulating oxide which enables the use of a thinner oxide layer and hence a smaller step, and the use of other metals which may have better step coverage. Problems with approaches such as these include filaments or metal remaining after the metal etch and failure problems after long term operation resulting from the thinner metallization on the small dimensioned contact sidewalls, however improved by the use of other metals.
It is the principal object of this invention to provide an improved process for making metal contacts for semiconductor integrated circuits. Another object is to provide improved semiconductor devices with metallization patterns, avoiding the problems of thinning at steps and contact apertures. A further object is to provide improved step coverage and lower resistance for metallization of semiconductor devices. Another object is to provide a method of making contacts that allow smaller pitch for the metal pattern (closer spacing between metal lines) due to smaller contact areas being allowed; thus higher density VLSI devices are possible.